Whenever a digital optical data signal is generated, transmitted, switched, multiplexed, demultiplexed, or otherwise processed, the signal invariably is subject to some degree of distortion. Distortion is typically cumulative and if the original signal is not periodically restored, data can become riddled with errors or become completely incomprehensible. Regenerators are utilized to provide this periodic restoration and restore the quality of the original data signal. Typically, regenerators are complicated and expensive opto-electronic devices, in which optical data is converted to an electrical signal, the signal is amplified and restored, and then the signal is converted back to an optical form.
Advances have recently been made in an attempt to produce an all-optical regenerator which does not require conversion of a data signal from optical to electronic and back to optical. In a published letter entitled "10 Gbit/s Soliton Data Transmission Over One Million Kilometres," M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, Electronics Letters 27, 1270-1272 (1991) a method is suggested in which in-line amplitude modulation of a signal at the bit-rate frequency acts as a regenerator distributed over the transmission line (local clock recovery is required in conjunction with this technique). A published letter entitled "All-Optical Signal Regenerator," J. K. Lucek and K. Smith, Optics Letters 13, 1226-1228 (1993) demonstrates a method for all-optical data regeneration (also requiring local clock recovery) utilizing a nonlinear optical loop mirror as described in "Nonlinear-Optical Loop Mirror," N. J. Doran and D. Wood, Optics Letters 13, 56-58 (1988). Utilizing the technique of soliton transmission with sliding-frequency guiding filters, as revealed in "The Sliding-Frequency Guiding Filter: An Improved Form of Soliton Jitter Control," L. F. Mollenauer, J. P Gordon, and S. G. Evangelides, Optics Letters 17, 1575-1577 (1992), the transmission line itself can be considered as an all-optical passive regenerator for a soliton pulse. This method is compatible with wavelength-division multiplexing and is characterized by an essentially ideal intensity transfer function, as described in "WDM Channel Energy Self-Equalization in a Soliton Transmission Line by Guiding Filters," P. V. Mamyshev and L. F. Mollenauer, Optics Letters 21, 1658-1660 (1996).
Therefore, there is a need for an all-optical digital data signal regenerator which does not require local clock recovery, and which is equally functional when utilized in conjunction with signal waveshapes other than solitons.